Energy from Gravity

November 9, 2012

Obtaining energy from gravity.

I am not talking about the change between potential and kinetic energy like in a damn but getting energy at the same level or height.

Who has not enjoy the wind just in front of a beach watching seagulls doing their apparently effortless fly. It seems like magic the way they hang up there in the air. Even more magic seems (at least to me) a 747 flying at low-speed just about to land. How this monster can hang up there? Even though I understand the principles of aerodynamics that explain it, I keep thinking that is almost poetry.

This poetry only works on Earth. In outer space, wings work only on movies. The wind energy that comes from the circular move of a wind turbine is in part due to gravity action over the air passing the turbine (see formula). Wind is a consequence of temperature differences in different points on Earth (also caused by rotation of Earth itself) but gravity is essential.

Wind Energy

The Maximum Theoretical Power (MTP) Output of a turbine is represented in the formula:

MTP = 1/2 * Air density * Turbine Swept Area * (Wind Speed) ^ 3

Air density depends on gravity. Air is thinner at 6000 meters than at 0 meters. The weight of all air above us increase the air pressure and also the density at sea level.

But the most important factor in the above formula is the wind speed.

If energy output depends on relative air speed; could we get energy from a vehicle due to its own speed, from the relative air speed created?

Is there any vehicle that gets energy from this relative air speed created?

The Greenbird has been capable of run at 126.1 mph with constant wind speed of 30 mph.

All energy has come from the wind especially from rest. But the gained speed also produced “apparent wind” (The wind that blows against you as a result of your passage through still air is known as the apparent wind. Read more: http://www.answers.com/topic/apparent-wind#ixzz21TXJ4jBi)

Resuming; the Greenbird has been capable of using its own speed in order to generate more energy and as a consequence more speed in a virtuous circle up to a 126 mph!

In sailboat racing, and especially in speed sailing, apparent wind is a vital factor, when determining the points of saila sail-boat can effectively travel in. A vessel traveling at increasing speed relative to the prevailing wind will encounter the wind driving the sail at a decreasing angle and increasing velocity. Eventually, the increased drag and diminished degree of efficiency of a sail at extremely low angleswill cause a loss of accelerating force. This constitutes the main limitation to the speed of wind-driven vessels and vehicles.

This image makes a good visual representation of forces on the vessel. The forward force will be reduced to zero if sail gets parallel to vessel direction of movement. Note that the sail force is still considerable at zero angle (while speed is still high) but in this vessel is not useful.

Once you know why Greenbird can’t go faster than 126 mph, the remarkable thing is that this vehicle is generating more energy from its own speed.

This is very Counterintuitive for any person that has taken a basic physics course. One of the first things that one learn in such a course is about drag.

It is a very well-known fact that a car needs more power from the engine as it increase the speed due to drag. As a matter of fact, drag gets incremented by the square of speed. So where comes the energy for the Greenbird to accelerate up to 126 mph?

Depending on form and other variables, there are very advance airfoil capable of having 100 times more lift than the drag they produce. A typical Cessna airplane can have 7 times more lift than drag. A Boeing 747, 17 times.

To measure lift and drag in airfoil, coefficients are used (Lift and Drag Coefficients). Lift to Drag ratio (L/D) is also a coefficient. The most advanced airplanes like the Virgin Atlantic GlobalFlyer can have a L/D coefficient of 37!

Another important factor is the “Aspect Ratio” or how long is the wing in proportion to the width. The longer the wing is, the better. The wings of the Virgin Atlantic GlobalFlyer have big “Aspect Ratios”.

So a wing with a good “Aspect Ratio” an a big L/D number can be considered an efficient wing. Creates big amount of lift with very low drag.

That is the secret of Greenbird incredible speed; a very efficient wing.

Greenbird is designed to use wind. It will not work even though we impulse it somehow in absence of wind.

A good question

Can we make a vehicle that can use relative wind speed in absence of wind?

The first thing I thought answering this question was: Why not install a wind turbine to a vehicle that use the electric energy to drive the wheels and start a virtuous cycle?

There are a lot of patents about this same idea. Much of those patents don’t take the “Thrust Force” that a typical wind turbine makes when they turn. “Thrust Force” is a force that pulls in the wind direction (then will be braking the forward movement of any vessel).

Big wind turbines design take this force into consideration to build the tower. Too much thrust and the tower can collapse.

In an airplane propeller the force that impulse the airplane is called thrust, as you can see in this diagram.

Thrust in an airplane propeller

Thrust is the force that impulse propeller planes

In a wind turbine is just the other way around as you can see in the following diagram.

Thrust force for a wind generator

Thrust in a wind turbine goes in the wind direction

This force would oppose the advance of our vehicle. So as we get speed, the turbine would oppose further with more force. The end result is a failed design.

A solution

It is clear that the Greenbird wing (or sail) produce enough force to reach 126 mph. At this speed almost all force is perpendicular (this is key) to the vehicle direction, or said it other way, it is wasting almost all wing energy produced. And still can go up to 126 mph! Just imagine the speed that could reach if it could use all the wing potential!

What if we connect several advance wings (like the ones in the Greenbird) around a single axle and connected to an electric generator? Remember all that wasted energy? Here we can use it to produce torque (remember that perpendicular force!) and then electricity.

You could think that this is very similar to a wind turbine but the main difference is the wing elements themselves and the “angle of attack” of the wings. Wings need to be long enough to produce torque in significant quantities and very low drag (angle of attack around 4 degrees) to get easy acceleration.

This proposed design turns very low if we compare it with a traditional wind turbine (and produce less energy) but drag is reduced even more.

The net result is a design that can be used in a vehicle impulsed by the generated energy.

Imagine a train with two generators attached:

Proposed train with two generators attached

This train obviously needs to be very well link to its track to withstand lateral wind.

Generators could be design to start at low speeds but then wings should be big enough (remember that wing lift is proportional to the square of speed). At the end is a compromise between diameter of generators and starting speed (speed that kick-start generators).

This train needs to be supplied with the energy to reach that speed (from batteries or from the track). After that, generators will produce the necessary energy to go on.

Resuming.

What is the relation between the Greenbird, the proposed train and gravity?

To begin with, atmosphere existence is due to gravity (as so many other things). Without air our generator will not work (nor any wind turbine). In outer space, acceleration is very easy to get due to lack of drag, but a wind turbine will not work out there.